1. Field of the Invention
The present invention relates; to an optical wavelength converter device, which converts a fundamental wave into a second harmonic, having a wavelength which is 1/2 of that of the fundamental wave; to a fiber-type optical wavelength converter device which converts a fundamental wave into a third harmonic; to a fiber-type optical wavelength converter device which converts fundamental waves having two different wavelengths into a wave having a frequency equal to the sum of or the difference between the fundamental waves; and to an optical wavelength converter module which employs such an optical wavelength converter device. More particularly, it relates to an optical wavelength converter device and an optical wavelength converter module, both of which can convert the conical wavefront of a wavelength-converted wave into a plane or spherical wavefront.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the wavelength of a laser beam into a second harmonic, i.e., shortening the wavelength of a laser beam, using nonlinear optical material. One well known example of such an optical wavelength converter device for effecting such laser wavelength conversion is the bulk crystal type converter device as disclosed in Introduction to Optical Electronics, pages 200-204, written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K. K.). This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, material which does not exhibit birefringence, or exhibits only small birefringence, even if it has high nonlinearity cannot be employed.
To solve the above problem, a fiber type optical wavelength converter device is proposed. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in the vol. 3, No. 2, pages 28-32, Bulletin of the Microoptics Research Group of a Gathering of the Applied Physics Society. Recently, many efforts are directed to the study of the fiber type optical wavelength converter device since it can easily gain phase matching between a guided mode in which a fundamental is guided through the core and a radiated mode in which a second harmonic is radiated into the cladding (the so-called Cherenkov radiation).
The wavelength-converted wave produced by any of the above optical wavelength converter devices is emitted from the output end of the cladding and has various applications. In many of the applications, the wavelength-converted wave is converged into a small spot. For example, the wavelength-converted wave is focused into a very small spot for higher recording density when a wavelength-converted wave is used optically to record information signals.
However, it is recognized that the wavelength-converted wave emitted from the fiber Cherenkov type optical wavelength converter device cannot converge into a small spot even if the emitted wave is passed through a general spherical lens.